Colorectal cancer (CRC) is the third most common cancer in U.S. About 65% of new CRCs and about 75% of CRC deaths occur among patients age 65 or older. About 40% of CRCs will relapse with metastatic disease or recurrence and many patients will die of their disease unless detected early enough to receive curative treatment. Several studies suggest a survival benefit of surveillance colonoscopy after curative surgery to detect local recurrence. Professional organizations recommend surveillance colonoscopy 1, 4, and 9 years after surgery (hereafter the guideline). The effectiveness of this guideline among the many CRC patients with additional chronic conditions (ACC) is unknown. ACC is defined as any additional chronic condition among CRC patients, also known as comorbidity, and are highly prevalent among CRC patients; over 65% of patients age 65 years or older have at least one ACC. ACCs could impact the effectiveness of surveillance colonoscopy by affecting the risk of recurrence or reducing life expectancy. Colonoscopy also carries significant risks, including cardiopulmonary complications, perforation, and even death. CRC patients with ACC (using a comorbidity index) are at increased risk of complications, which may be related to the preparation, sedation, or the colonoscopy procedure itself. Our overarching goal builds upon these findings to determine differences in safety and effectiveness of the guideline among elderly CRC patients with specific ACCs that will lead to tailored follow-up strategies for these patients by refining clinical decision making designed to maximize prognosis, reduce complications, and reduce cost. Specific ACC are selected based on their risk on CRC recurrence and death, and prevalence in CRC patients. Aim 1. Describe patterns of use of surveillance colonoscopy among CRC patients with specific ACCs. Aim 2. Describe and compare the incidence of complications resulting from surveillance colonoscopy among elderly CRC patients with specific ACCs. Aim 3. Describe and compare the effectiveness of surveillance colonoscopy among CRC patients with specific ACCs, particularly their risk of disease relapse, surgical resection, and death. We will use the 1999-2011 large, high-quality, population-based, real-world, linked Surveillance, Epidemiology, and End Result (SEER)-Medicare data and advanced methods to compare differences in the safety and effectiveness of the guideline among CRC patients aged 66 or older with ACC. This amended application could have a huge impact on patient outcomes, real-world clinical care practices, local health care resources, and financial costs, and advance the field of comparative effectiveness research in CRC patients with ACC given the large and growing number of CRC patients undergoing postoperative surveillance. It also will provide extensive data for ample future projects.